General Information for Tennis Players
With time and use, strings not only break but also lose the properties which make them 'playable'.
Physical wear, in the form of notching and fraying, are the most obvious signs your racket needs a restring prior to actual breakage. A less noticeable effect is 'string creep' - a gradual loss of tension with age.
However, strings have one other major playability factor which is usually overlooked, and which deteriorates with age irrespective of either physical wear or tension loss - elasticity.
It is the elastic nature of the string which generates power in the form of recoil from impact. It happens at a molecular level, as the particles which make up the string slide past each other and then return after each impact event. However, over time, repeated impacts and constant tension cause the molecules to slide beyond the point of no return, and so the string 'ages' and loses elasticity and its ability to generate power.
You will notice this as a 'dead' feel to the strings. You should use your own intuition to identify when it happens, because it occurs regardless of actual string tension, and there is no specific way of measuring it whilst the strings are still in the racket.
As a rule of thumb, you should restring as often each year as you play each week, or every 12 months, whichever is the more frequent.
Stringing to avoid Injury
If you suffer from wrist or elbow problems, avoid the polyester monofilaments and Aramid composites. These are hard and unforgiving strings even when combined with softer strings as a hybrid.
The softest and most playable string is Natural Gut, but its expense takes it out of reach of all but the most serious (or wealthy!) players. Otherwise, look at the multifilament strings which are renowned for their softness and playability - such as synthetic gut.
If your problems are not too serious, you might try a hybrid using a 'soft' multifilament with one of the 'hard' co-polyester monofilaments. Many professional players such as Jennifer Capriati, Novak Djokovic and Roger Federer use this type of combination.
Stringing to generate spin
It was originally thought that spin was best generated by a static stringbed with a textured surface - the common-sense theory being that the rough, hard surface sliding over the ball would cause it to spin the most. However, the use of high-speed cameras and advances in measurement technology have shown this to be incorrect.
Spin is actually generated mostly by the ball causing the strings to slide sideways upon ball impact, followed by 'snap-back' of the strings as the ball starts to leave the stringbed. This movement of the returning strings imparts the spin-inducing torque to the ball. (In fact, if the strings do not move during impact, then all strings of all types generate roughly the same amount of spin!)
Low-friction string surfaces
The lower the friction between them, the more the main strings will stretch and slide, and the more readily they will snap-back to produce spin.
Hybrids with gut mains and polyester crosses have some of the lowest string-to-string friction levels. When the main is softer than the cross, it will deform over the cross such that it forms a slot or groove to move over the cross. This will facilitate movement, especially if the deformation causes a leakage of some oil or lubricant from inside the string, which is the case with Natural Gut.
Interestingly for those interested in experimentation, if you change the combination to gut crosses and polyester mains string-to-string friction levels are some of the highest! If the cross is softer than the main, the main will sink into the cross. To initiate movement, the main will first have to climb a hill out of the valley it sits in, and then continually plow its way forward down the soft length of the cross. Also, if the cross is a multifilament string like gut, it can fray and break filaments as the polyester main moves over its length, and these broken filaments can stick up as obstacles to slow movement over them.
Textured string surfaces
Interestingly, the same texture which has no effect on spin with a static stringbed has a great influence on a moving stringbed. During snap-back, textured strings add further spin because they increase the friction between the moving string and the ball.
Polyesters have been shown to add about 20% more spin than nylons, although individual polyesters can differ up to about 15% from each other in spin production. Nylons vary by as much as 20% from each other, so there is an overlap of properties. However, the difference between the spiniest polyester and the stickiest nylon is almost 50% - so the textured polyesters will generate the most spin.
Thinner strings will produce more spin than thicker strings, all other properties being equal. This is primarily because the smaller surface area of contact between the strings has less friction, which allows a greater level of snap-back.
It has been demonstrated that an open string pattern e.g. 16x18 will generate more spin than a dense string pattern e.g. 18x20. This is due to the simple fact that in an open pattern the main strings are less constrained, and better able to stretch, move and snap-back. Unfortunately, the downside of this is less durability of the strings, since greater movement = greater wear, and there are fewer strings to absorb each impact of the ball.
Lower tensions are better for creating spin, since not only can the strings slide over each other more easily, but the ball sinks deeper into the trampoline-like stringbed, allowing the strings to have more effect on the ball.
Stringing for power
Generally, power is a feature of string tension rather than type. However, the following do have influence:
An open pattern generates more power than a dense pattern (fewer strings stretch more easily).
Thinner string generates more power (thinner string stretches more easily).
More elastic strings generate more power (and are also better at shock absorption) due to a greater 'trampoline' effect.
Stringing for Playability or Durability
Unfortunately, each of these features requires the opposite string properties of the other.
Thinner strings offer a greater level of playability, touch, feel and spin, whilst thicker strings are the more durable. Natural Gut and its synthetic counterparts are the most playable strings, whilst some of the Aramid (Kevlar-type) composites are the hardest and most durable.
Hybrid String Combinations
Hybrids are becoming increasingly popular in tennis as players strive to find the perfect stringbed for their style of play. There are several reasons for choosing a hybrid string combination:
1. Increased durability. The main strings bear the wear load. These are the strings which move when the ball is hit, and are consequently 'sawed' by the cross strings. A highly durable main string can be mixed with a more playable cross string. This combination favours durability over playability. A thinner gauge mainstring can also be mixed with a thicker gauge cross for the same effect.
2. Increased playability. The main strings also confer the major 'playability' factor to the stringbed, so a very playable mainstring can be combined with a more durable cross-string. This combination favours playability over durability.
3. Increased tension stability. Over time, strings lose tension by stretching. As the tension drops, the racket will lose both feel and control. At the same time, because the strings have lost their elasticity, power output will drop. Eventually, the stringbed becomes 'dead' and requires replacement. Again, the main strings confer most tension stability, and the use of a tension-holding mainstring with a cross-string for either durability or playability will create a versatile but long-lasting stringbed.
4. Generation of spin. As we have described above, the properties of different strings can enhance the amount of spin generated by the stringbed.